This application claims the priority of 101 46 643.9, filed Sep. 21, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process of calibrating the radar signals at the subapertures of the antenna of a two-channel SAR (Synthetic Aperture)/MTI (Moving Target Indication) radar system.
In a two-channel radar system, the two subaperture channels (1, 2) of an antenna (A), in the antenna front end, can be combined by means of a wave guide element (T) (normally a so-called MagicT), to a sum channel (xcexa3) and a difference (xcex94) channel, in a known manner. Such an arrangement is illustrated in FIG. 1, which also shows that the sum (xcexa3) and difference (xcex94) channel signals are fed to analog and digital signal processors (AS, DS).
In the digital signal processing of an SAR radar, the sum channel signal is used to establish so-called radar ground maps. In the digital signal processing of an MTI radar, the sum channel signal is used to detect and indicate moving targets in images similar to radar ground maps. For this purpose, FIG. 2 shows a typical distance Doppler image in the sum channel after the detection of fixed and moving targets generated by means of the MTI method. In this case, the signal powers are shown with respect to the distance gates and the normalized frequency f/fs. As can be seen, the area of the antenna spot (light area), also called the major lobe clutter, is shifted with respect to the zero Doppler frequency position because of a relative geometry change during the illumination time. The circles illustrated in the picture indicate the detected fixed and moving targets. In FIG. 2, the Doppler frequency position of the major lobe clutter is, for example, at a normalized frequency of 0.2.
While, in SAR radar signal processing, the difference channel signal is not used any further, in the corresponding MTI radar signal processing, the difference channel signal is required, on the one hand, to separate fixed and moving targets and, on the other hand, to reposition the moving targets shifted with respect to the Doppler frequency.
One method for the fixed target suppression is known as STAP (Space Time Adaptive Processing) method. In order to maximize moving target suppression, the subaperture channel signals must be exactly identical in amplitude and, differ in phase only by a value caused by the relative geometry and antenna arrangement. For moving target repositioning, it is also necessary to compensate various phase differences between the subaperture channel signals which result in faulty positioning. A conventional method of determining the compensation factors of the amplitude and the phase is to use defined test signals which are fed into the wave guide element, or are irradiated by way of the antenna. This takes place before the actual use of the radar antenna. The sum and difference channel signals behind the wave guide part are then computed back into the input signals in front of the wave guide part, and thus into the signals of the two subaperture channels. If the correction factors were determined correctly, the two signals will be identical. The factors are filed in the memory of the digital signal processing and, during the use of the radar antenna, are read out of this memory. Such a method was described by Shunjun W. et al. in xe2x80x9cAdaptive Channel Equalization for Space-Time Adaptive Processingxe2x80x9d; IEEE International Radar Conference 1995.
The computation of the signals of the subaperture channels normally takes place according to the following equations:                                           x            1                    ⁡                      (            t            )                          =                                                            x                s                            ⁡                              (                t                )                                      -                                          x                d                            ⁡                              (                t                )                                              2                                    (a)                                                      x            2                    ⁡                      (            t            )                          =                                                            x                s                            ⁡                              (                t                )                                      +                                          x                d                            ⁡                              (                t                )                                              2                                    (b)            
wherein
x1(t), x2(t): time signal of the two subaperture channels,
xs(t), xd(t): time signal sum and difference channel.
In this case, it is disadvantageous that this method of operation has to be implemented beforehand. Moreover, the amplitude and phase tracking differences which occur during the running operation can no longer be corrected. It is also a disadvantage that testing signals have to be fed in the running operation, which leads to higher expenditures and to a possible interruption of the coherent digital signal processing.
It is an object of the invention to provide an improved process for computing the sum and difference channel signal back to the signal existing at the output of the two subapertures.
Another object of the invention is to provide a process which does not require a test signal, during the running operation; adaptively computes the correction factors from the radar signals for generating the picture; and carries out the correction.
These and other objects and advantages are achieved by the calibration process according to the invention, in which the signals of the two subaperture channels are computed from the amplitude- and phase-shifted sum and difference channel signal according to the following equations:                                           X            1                    ⁡                      (                          r              ,              f                        )                          =                                                            X                S                            ⁡                              (                                  r                  ,                  f                                )                                      -                                                                                X                    D                                    ⁡                                      (                                          r                      ,                      f                                        )                                                  ·                exp                            ⁢                              xe2x80x83                            ⁢                              (                                                      -                    j                                    ⁢                                      xe2x80x83                                    ⁢                                      Φ                    0                                                  )                                              2                                    (        1        )                                                      X            2                    ⁡                      (                          r              ,              f                        )                          =                              a            0                    ·                      (                                                                                X                    S                                    ⁡                                      (                                          r                      ,                      f                                        )                                                  +                                                                            x                      D                                        ⁡                                          (                                              r                        ,                        f                                            )                                                        ·                                      exp                    (                                                                  -                        j                                            ⁢                                              xe2x80x83                                            ⁢                                              Φ                        0                                                                                                        2                        )                                              (        2        )            
wherein
X1(r,f), X2(r,f): Frequency spectrum of the two subaperture channels,
XS(r,f),XD(r,f): Frequency spectrum of the sum and difference channel,
r: distance cell,
f: Doppler frequency,
"PHgr"0: phase correction factor,
a0: amplitude correction factor.
By means of the phase correction factor "PHgr"0, the phase difference is determined between the sum and difference channel.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.